The purpose of this study is to investigate 1) the effects of various dietary treatments on disease progression in the glycogen storage diseases (GSD), 2) the molecular basis and genotype-phenotype correlations of glycogen storage disease, 3) the mechanism and treatment of renal disease associated with type I GSD, and 4) development of recombinant enzyme replacement therapy for GSD type II. Methods: The goals of dietary treatment are to achieve maximum growth potential and/or to improve muscle strength. Patients are seen at 3 to 6 month intervals for diet evaluation, anthropomorphic measurements and serum chemistries. Muscle strength is evaluated by formal muscle testing including Cybex, by EMG, and by changes in serum enzymes (CPK, LDH an isoenzymes, SGOT, SGPT). Renal function, clearances, and factors affecting hyperfiltration are monitored with diet intervention. Changes in size and number of hepatic adenomas are evaluated at 6 month intervals by ultrasound and CT, and at 3 to 4 year intervals by liver scan. Alpha-1-Fetoprotein is determined every 6 months in patients with adenomas. Yearly echocardiograms and EKG's are performed in adults with GSD Types III and V, and in all patients with GSD Type II. Baseline data on muscle strength and EMG is obtained. Dietary instruction is given by the CRU dietitian, and the patient is re-evaluated at 3 to 6 month intervals. Skin fibroblasts and lymphoblastoid cells are obtained from GSD Type III patients for analysis of debrancher enzyme activity, antibody-reactive materials, and DNA levels.